The term structural foam is used to refer to a plastic product having an intregral skin, a cellular core, and having a high enough strength-to-weight ratio, depending upon the application, to be classed as "structural." An almost endless variety of moldable resins, including thermoplastic as well as thermoset polymers, can be employed for forming structural foam from a variety of processes. Material selection is ordinarily based upon the size and the required strength the article to be formed, the desired physical characteristics of the article, and economy of fabrication. Because of the desirably high strength-to-weight ratio which can economically achieved with the use of structural foam, this form of a material has been advantageously employed for fabricating products of almost every conceivable description.
The cellular nature of structural foam is achieved by the introduction of a so-called "blowing agent" into plastic material, typically during an injection molding type process. The blowing agent may be introduced in a gaseous form (typically nitrogen) into the plastic material sometime during the molding process, or may be introduced in the form of a liquid or solid gas-generating chemical agent prior to or during the molding process. Such chemical blowing agents are typically heat-reactive, with the release and dispersion of gas in the plastic material being a function of the material's temperature and pressure. Similarly, techniques are known by which a blowing agent in a gaseous form is introduced and dispersed into the plastic material during the molding process.
One typical apparatus for injection molding structural foam includes a plasticating device (such as an extruder) for receiving plastic material, ordinarily in pellet form, and for heating the plastic material to a molten flowable state. The flowable plastic material is moved from the extruder to an accumulator, with the accumulator thus receiving a predetermined quantity, sometimes referred to as a "shot" or "charge," of the flowable plastic material.
The accumulator is in communication with a valve-like nozzle assembly positioned just upstream of a mold assembly in which the article to be formed is shaped. After filling of the accumulator, the nozzle assembly is opened in timed relation to operation of the accumulator, with the accumulator operated to drive the molten plastic charge therein through the nozzle and into the mold cavity defined by the mold assembly. The material is driven under relatively high pressure, with the predetermined quantity of the material to be introduced into the mold being received therein rather quickly, usually in a matter of a few seconds or less. Ordinarily, the mold cavity is only partially filled with the plastic material. Formation of the cellular core of a structural foam article takes place within the mold as the blowing agent dispersed within the plastic material expands and causes the formation of relatively large gas bubbles within the plastic material under the relatively reduced pressure of the mold cavity.
In most applications, it is highly desirable to form a structural foam article with a thin, smooth, solid, swirl-free, unbroken surface or skin, and with a cellular core which only very gradually increases in density toward the integral skin. Effective density control is essential to cost-effective fabrication of an an article from structural foam. Because different plastic materials exhibit widely different physical characteristics (i.e., density, apparent viscosity, etc.), many different chemical blowing agents have been developed in an effort to permit fabricators of structural foam articles to form any particular article with optimum efficiency and control. Likewise, various techniques have been developed for the introduction of gaseous blowing agents into the plastic material to provide the desired controlled expansion of the gas for formation of the cellular core of a structural foam article.
Effective control of the "foaming" or expanding action of the blowing agent in a plastic material has proven crucial to cost-effective fabrication of structural foam articles having the desired quality and physical characteristics. Therefore, pressures, temperatures, and like parameters must be very carefully monitored and controlled during the structural foam molding process. Additionally, it has been recognized that effective control of the dispersion and distribution of the blowing agent in the plastic material greatly facilitates control of the foaming action. This is one reason why the blowing agent for structural foam (either gaseous or chemcial) has been typically introduced into the plastic material at or before the extruder which plasticates and melts the material to render it flowable, since the extruder acts to mix and disperse the blowing agent into the plastic material. Additionally, the extruder of a system is ordinarily run on a continuous basis, while other components of the system, such as the accumulator for example, are cyclically operated.
As will be recognized, the extruder is ordinarily the component of a molding system which is most far-removed from the mold assembly within which the blowing agent in the plastic is intended to expand. It is this fact which creates one of the most difficult to solve problems in effectively controlling the foaming action of structural foam, particularly when the blowing agent is introduced in a gaseous form. Specifically, it is highly desirable to maintain the blowing agent in as finely dispersed form as possible. Therefore, gaseous blowing agents are typically introduced in the form of microscopic bubbles as the plastic material is advanced through the extruder. As a result of material characteristics, some of the gas may be in solution with the polymer while the rest is merely a two-phase mixture. However, almost as soon as the gaseous blowing agent is introduced into the plastic material, the "microbubbles" tend to migrate together to form undesirably large gas bubbles lacking uniformity of size and distribution. This tendency of the blowing agent to migrate out of the plastic material is particularly exacerbated as the material is mechanically "worked" during passage into and out of the accumulator, and as the material is driven from the accumulator through the molding nozzle into the mold assembly. Naturally, introduction of the blowing agent into the plastic material as close as possible to the mold assembly minimizes the working of the material prior to its introduction into the mold, but problems of space limitation and effective blowing agent dispersal have heretofore resulted in only limited success with efforts to perfect such techniques.
In view of the above, numerous attempts have been made in the past to enhance the dispersion and distribution of the blowing agent in the plastic material prior to introduction into the mold assembly. These attempts have included the provision of mixing devices in the flow path of the plastic material as it travels to the mold assembly.
One type of mixing device heretofore employed for this purpose is a so-called static mixer which is fixedly mounted in the plastic flow. Such devices were originally conceived for enhancing the dispersion of dyes or pigments in the plastic material, but their use was found to enhance dispersion of a blowing agent in flowable plastic. Static mixers have been used in various forms, including mesh screens, auger-like helically-arranged vanes, and other configurations, in an attempt to maintain the blowing agent in the plastic material in as finely dispersed state as possible. However, the use of such static mixers has only provided very limited success in solving the problem of maintaining the uniform and fine dispersion of the blowing agent in the plastic material. For example, the disposition of a mesh screen in the flow path of the plastic material having a sufficiently fine mesh to achieve the desired blowing agent distribution creates unacceptably high resistance to the plastic flow.
Recognizing that powered agitation or mixing of the plastic material might enhance blowing agent distribution, workers in the art have in the past attempted to perfect an externally-powered mixing device. However, the problems encountered in practically implementing such a device have proved virtually insurmountable. In order to perform its intended function, a mixing device is perferably positioned as close to the mold assembly as is practicable. However, space limitations become critical, particularly in view of the amount of power required to drive a mixing device for effective blowing agent dispersion. Additionally, such an externally-powered mixer undesirably introduces energy into the molten plastic material, affecting its temperature and viscosity (which as noted, must be very carefully controlled during the molding process). Further, since such a mixing device is preferably positioned downstream of the system's accumulator, plastic flow to the device is cyclic. Therefore, to avoid excessively working the plastic material at the mixer when the material is not flowing, it becomes necessary to operate the mixing device on a non-continuous, carefully timed basis.
In view of the above design considerations, embodiments of an externally-powered mixing device have been undesirably complex, cumbersome, and expensive. High external power requirements and space limitations have mandated resort to relatively complicated external driving means, with a complex control system further required to effect properly timed cyclic operation. The difficulties encountered in implementing such an arrangement have thus far prevented development of a commercially viable externally-powered mixing device.
From the foregoing discussion, it will be apparent that the development of a practical mixing device for effecting and/or enhancing the dispersion of either a gaseous or chemical blowing agent in a plastic material represents a very significant advance in the art of structural foam molding.